Treatment of Plastic Surfaces After Etching in Nitric Acid Containing Media

ABSTRACT

A process for plating metal on plastic substrates, particularly ABS substrates, without the use of chrome containing etchants is disclosed. The process involves (i) etching the plastic substrate in an acidic solution of nitrate ions, and preferably silver ions, (ii) conditioning the substrate in an aqueous solution containing an amine or ammonia, (iii) activating the substrate, preferably with a palladium activator, and (iv) plating the substrate with an electroless plating solution. The process allows for complete adherent electroless plating of plastic substrates, particularly ABS substrates, without the use of chromic etchants.

FIELD OF THE INVENTION

The present invention relates generally to the treatment of plasticsurfaces following etching in an acidic solution containing nitrateions.

BACKGROUND OF THE INVENTION

For many years, processes have been available to facilitate thedeposition of electrodeposited metals onto plastic substrates. A typicalprocess involves the steps of:

-   -   (1) etching the plastic in a suitable etching solution such that        the surface of the plastic becomes roughened and wetted so that        the subsequently applied deposit has good adhesion;    -   (2) activating the surface of the plastic using a colloidal or        ionic solution of a metal capable of initiating the deposition        of an autocatalytically applied metal coating of typically        copper or nickel;    -   (3) depositing a thin layer of autocatalytically applied metal;        and    -   (4) carrying out electrodeposition of metal onto the metallized        plastic substrate.        Typically, layers of copper, nickel and/or chromium are applied        to produce the final article.

The most widely used plastic substrates includeacrylonitrile/butadiene/styrene copolymers (ABS) or ABS blended withpolycarbonate (ABS/PC). These materials are readily formed intocomponents by the process of injection molding. ABS comprises arelatively hard matrix of acrylonitrile/styrene copolymer and thebutadiene polymerizes to form a separate phase. It is this softer phaseof polybutadiene (which contains double bonds in the polymer backbone)which can be readily etched using various techniques.

Traditionally, the etching has been carried out using a mixture ofchromic and sulfuric acids which must be operated at an elevatedtemperature. The chromic acid is capable of dissolving the polybutadienephase of the ABS by oxidation of the double bonds in the backbone of thepolybutadiene polymer, and this has proven to be reliable and effectiveover a wide range of ABS and ABS/PC plastics. However, the use ofchromic acid is becoming increasingly regulated because of its toxicityand carcinogenic nature. For this reason, there has been a considerableamount of research into other means of etching ABS and ABS/PC plastics.

There are a number of approaches possible in order to attempt to achievethis. For example, acidic permanganate is capable of oxidizing thedouble bonds in the polybutadiene. Chain scission can then be achievedby further oxidation with periodate ions. Ozone is also capable ofoxidizing polybutadiene and this approach has also been attempted.However, ozone is extremely dangerous to use and is also highly toxic.Likewise, sulfur trioxide can be successfully utilized to etch ABS, butthis cannot be successfully achieved on a typical plating line. Otherexamples of prior art techniques for etching ABS plastics without theuse of chromic acid can be found in U.S. Pat. Pub. No. 2005/0199587 toBengston, U.S. Pat. Pub. No. 2009/0092757 to Sakou and U.S. Pat. No.5,160,600 to Gordhanbai, the subject matter of each of which is hereinincorporated by reference in its entirety. However, none of thesemethods have achieved widespread commercial acceptance.

Thus, there remains a need in the art for an improved process of etchingplastics without chromic acid, while continuing to utilize aconventional activation process containing a palladium colloid followedby electroless nickel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process foretching plastics without the use of chromic acid.

It is another object of the present invention to provide a process foretching acrylonitrile/butadiene/styrene copolymers without the use ofchromic acid.

It is still another object of the present invention to provide animproved conditioning treatment for conditioning the surfaces of anetched plastic.

To that end, the present invention relates generally to a method oftreating a plastic substrate to accept electroless plating thereon, themethod comprising the steps of:

-   -   a) etching at least a surface of the plastic substrate by        contacting the plastic substrate with an acidic solution        containing nitrate ions;    -   b) contacting the etched plastic substrate with a conditioning        solution comprising an aqueous solution comprising ammonia, an        amine or combinations thereof;    -   c) activating the etched and conditioned plastic substrate; and    -   d) immersing the activated plastic substrate into an electroless        metal plating solution to deposit metal thereon.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts an infra-red analysis obtained from untreated ABS.

FIGS. 2A and 2B depict an infra-red analysis obtained from ABS treatedwith a chromic acid/sulfuric acid etch solution of the prior art.

FIGS. 3A and 3B depict an infra-red analysis obtained from ABS treatedwith an acidic solution of nitrate and silver ions.

FIGS. 4A and 4B depict an infra-red analysis obtained from ABS treatedwith an acidic solution of nitrate and silver ions and then post-treatedin an ammonia solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In preliminary experiments using nitric acid/silver(II) etchcompositions, the inventors of the present invention discovered thatalthough this etch composition can be used to effectively etch an ABS orABS/PC plastic to give an excellent surface topography, the subsequentcatalysis of the surface could not be achieved and there was nodeposition of nickel when the components were subsequently immersed inthe electroless nickel plating process. Examination of the surface ofthe plastic using infrared spectroscopy indicates that the surface ofthe plastic has been chemically altered to some degree. New peaks werefound following the etching stage, which almost disappeared when theplastic was immersed in hot water (80° C.) for 10 minutes. However, eventhough the surface of the plastic had apparently reverted in compositionto something similar to its original composition, palladium adsorptionand subsequent catalyzation of the surface could not be achieved.

Surprisingly, the inventors of the present invention have discoveredthat immersing the etched plastic in a solution containing amines cancondition the surface so that palladium adsorption can be achieved.Without wishing to be bound by theory, the inventors consider that it ispossible that the amines are adsorbed onto the surface of the etchedplastic, thus imparting a positive charge on the surface of the plasticwhen immersed into the acidic palladium colloid solution. With primary,secondary and tertiary amines, this positive charge is most likelyformed by protonation of the amines, and with quaternary amines, thepositive charge is already present on the amine.

The inventors have also found, through infrared spectroscopy, that anetched plastic can be modified with an amine-based post treatment. Inaddition to the disappearance of the peaks introduced following theetching stage, a new functional group appears to have been introduced.The composition of the invention conditions the surface of the plasticso that effective palladium adsorption can be achieved in order tocatalyze the subsequent deposition of autocatalytic metal deposits.

According to the present invention, a method is provided for thecatalysis and subsequent metallization of plastics which have beenetched in nitric acid containing solutions. In a preferred embodiment,the method of the invention comprises the following steps:

-   -   a) etching at least a surface of the plastic substrate by        contacting the plastic substrate with an acidic solution        containing nitrate ions;    -   b) contacting the etched plastic substrate with a conditioning        solution comprising an aqueous solution comprising ammonia, an        amine or combinations thereof;    -   c) activating the etched and conditioned plastic substrate; and    -   d) immersing the activated plastic substrate into an electroless        metal plating solution to deposit metal thereon.        Following the above steps, the metallized component can be        electroplated in the usual manner.

The acidic etching solution preferably comprises nitric acid. Inaddition, other mineral acids such as sulfuric acid may also be added tothe composition. In a preferred embodiment the acidic etching solutionalso contains oxidizing metallic ions of metals including, for example,silver, manganese, cobalt, cerium and combinations thereof, preferablyin their highest oxidation state. Preferably, these ions are produced bya process of electrochemical oxidation. In addition, if desired, awetting agent may also be added to the acidic etching solution. Onesuitable wetting agent is available from MacDermid, Inc. under the tradename Macuplex STR.

Thereafter, the etched plastic substrate is contacted with theconditioning solution. In one preferred embodiment the etched plasticsubstrate is immersed in the conditioning solution. The concentration ofamines or ammonia in the aqueous conditioning solution is not criticalbut is preferably within the range of about 5 to about 100 g/L, morepreferably in the range of about 10 to about 50 g/L. The pH of thesolution may be from 0 to 14, but is preferably in the range of 6-12.

As discussed above, the amine may be a primary, secondary, tertiary orquarternary amine. In the alternative, the solution may comprise ammoniainstead of the amine. In addition, it is also possible to use acombination of different amines or a combination of an amine withammonia in the conditioning solution of the invention.

Suitable primary amines include, for example, monoethylamine,mono-n-propylamine, iso-propylamine, mono-n-butylamine, iso-butylamine,monoethanolamine, neopentanolamine, 2-aminopropanol, 3-aminopropanol,2-hydroxy-2′(aminopropoxy)ethylether, 1-aminopropanol,monoisopropanolamine, diethylaminopropylamine, aminoethyl ethanolamineand combinations of the foregoing. In a preferred embodiment, theprimary amine comprises monoisopropanolamine or diethylenetriamine.

Suitable secondary amines include, for example, diethylamine,dibutylamine, diethanolamine, methylethylamine, di-n-propanolamine,di-iso-propanolamine, N-methylethanolamine, N-ethyl ethanolamine,N-methyl ethanolamine, di-isopropanolamine, diethylenetriamine,triethylenetetramine, tetraethylenepentamine and combinations of theforegoing. In a preferred embodiment, the secondary amine comprisesdi-ethanolamine or diethylenetriamine.

Suitable tertiary amines include, for example, N,N-dimethylethanolamine,triethylamine, trimethylamine, triisopropylamine, methyldiethanolamine,triethanolamine, and combinations of one or more of the foregoing. In apreferred embodiment, the tertiary amine comprises N,N-dimethylethanolamine.

Quarternary amines are also generally suitable, including quarternary(poly) amines. Suitable quarternary amines also include polymericquarternary amines having the general formula:

Wherein:

R¹, R², R³ and R⁴ independently can be the same or different and may beselected from —CH₃, —CH₂CH₃, —CH(CH₃)₂ or —CH₂CH₂OH;

R⁵ is —CH₂CH₂—, —CH₂CH₂CH₂—, —CH₂CH₂CH₂CH₂—, —CH₂CHOHCH₂— or—CH₂CH₂OCH₂CH₂;

X and Y can be the same or different and are selected from Cl, Br, andI;

v and u can be the same or different and each can be 1 to 7; and

n is 2 to about 200.

In one embodiment the polymeric quaternary amine is Mirapol™ WT(available from Rhodia) in which in the above formula:

R¹, R², R³ and R⁴ are each CH₃;

R⁵ is —CH₂CH₂OCH₂CH₂;

v and u are 3;

X and Y are Cl; and

n is an average of about 6.

Other suitable polymeric amines include polyethyleneimines such asLugalvan™ G35 available from BASF.

The invention can now be illustrated by reference to the followingnon-limiting examples:

The following details apply to the examples:

POLYLAC™ PA727 is a commercial grade of acrylonitrile butadiene styrene(ABS) manufactured by Chi Mei, Inc., Taiwan.

The following products described in the examples are available fromMacDermid, Inc. and were used in accordance with their product datasheets.

Product Description ND7 Supreme ™ Pre-etch cleaning/degreasing solutionMacuplex ™ STR Wetting agent Macuplex ™ 9338 Neutralizing treatment forhexavalent chromium Macuplex ™ D-34C Activating solution based oncolloidal suspension of metallic palladium particles in a stannouschloride medium Macuplex ™ 9369 Post-activator acidic treatment toimprove the autocatalytic nature of the activated surface Macuplex ™J-64 Electroless nickel plating bath

-   Infra-red (IR) instrument and analysis details:-   Instrument—PerkinElmer spectrum 100 FTIR spectrometer-   Analysis details:    -   Attenuated total reflectance (ATR) mode    -   Wavenumber range—4000-6000 cm⁻¹    -   Number of scans—8

EXAMPLE 1

-   Untreated POLYLAC™ PA727-   The Infra-Red analysis obtained from untreated ABS is shown in FIG.    1.

EXAMPLE 2

-   POLYLAC™ PA727 processed through a chromic acid/sulfuric etch    solution of the prior art.

Temperature Process Details (° C.) Time ND7 Supreme ™ 50 2 minutes Waterrinse Ambient 1 minute  Etch 4M chromic acid 68 7 minutes 3.6M sulfuricacid 1 ml/l Macuplex STR Water rinse Ambient 2 minutes Macuplex ™ 933830 2 minutes Water rinse Ambient 1 minute  Dry Infra-Red analysis

The infra-red spectrum obtained is shown in FIGS. 2A and 2B.

In the following examples 3-9, the chromium free etch solution iscontained in a 2-compartment glass cell separated by a glass frit, withthe etching solution being the anolyte (the catholyte being of the samecomposition with the exception of the silver nitrate being absent in thecatholyte). The anode and cathode materials were platinized titaniummesh and the anodic current density used was 32.5 mA/cm². This systemwas used to electrochemically oxidize the silver ions to the +2oxidation state.

A mechanical stirrer bar was used to provide agitation and the cell waselectrolyzed by a minimum of two hours before use in order to generate asignificant quantity of silver (II) ions.

Example 3

Example 3 illustrates an ABS substrate processed through non-chrome etchsolution:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50 2 minutes Waterrinse Ambient 1 minute  Etch 0.1M AgNO₃ 50 6 minutes 9M HNO₃ 7 ml/lMacuplex STR Water rinse Ambient 2 minutes Dry Infra-Red analysis

The infra-red spectrum obtained is shown in FIG. 3A and 3B. FIGS. 3A and3B depict the FTIR spectrum of POLYLAC™ PA727 etched in an acidicsolution containing nitrate ions and silver ions. FIG. 3A shows theresults at 4000-600 cm⁻¹ and FIG. 3B shows the results at 2000-600 cm⁻¹.The “*” in the FIGS. 3A and 3B indicate peaks that have appeared due tothe etching process.

EXAMPLE 4

Example 4 illustrates an ABS substrate processed through a non-chromeetch solution and an ammonia post-treatment solution.

Temperature Process Details (° C.) Time ND7 Supreme ™ 50 2 minutes Waterrinse Ambient 1 minute  Etch Same as Example 3 50 6 minutes Water rinseAmbient 2 minutes Ammonia treatment 40 ml/l of 0.880SG 50 5 minutesammonia solution (or 0.68M NH₃) Water rinse Ambient 1 minute  DryInfra-Red analysis

The infra-red spectrum obtained is shown in FIGS. 4A and 4B. FIGS. 4Aand 4B depict the FTIR spectrum of POLYLAC™ PA727 etched in an acidicsolution containing nitrate ions and silver ions and post treated in anammonia solution. FIG. 4A shows the results at 4000-600 cm⁻¹ and FIG. 4Bshows the results at 2000-600 cm⁻¹. As can be seen in FIGS. 4A and 4B,there is an absence of the peaks introduced in Example 3. In thisexample, the “*” in FIG. 4B indicates a new peak introduced by treatmentwith an amine.

EXAMPLE 5

Example 5 illustrates an ABS substrate processed through a non-chromeetch solution an ammonia post-treatment solution and up to theelectroless nickel stage:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50 2 minutes Waterrinse Ambient 1 minute  Etch Same as Example 3 50 6 minutes Water rinseAmbient 2 minutes Ammonia treatment 40 ml/l of 0.880 SG 50 5 minutesammonia solution Water rinse Ambient 1 minute  Acid rinse 2.8M HClAmbient 1 minute  Macuplex ™ D-34C 27 3 minutes Water rinse Ambient 1minute  Macuplex ™ 9369 48 2 minutes Water rinse Ambient 1 minute Macuplex ™ J-64 32 7 minutes Water rinse Ambient 1 minute  Dry

The result was full electroless nickel metallization.

EXAMPLE 6

Example 6 illustrates an ABS substrate processed through a non-chromeetch solution, deionized water post-treatment solution and up to theelectroless nickel stage:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50  2 minutesWater rinse Ambient 1 minute Etch 0.1M AgNO₃ 55  6 minutes 6M HNO₃ 6MH₂SO₄ 1 ml/l Macuplex STR Water rinse Ambient  5 minutes Hot water rinse80 10 minutes  Water rinse Ambient 1 minute Acid rinse 2.8M HCl Ambient1 minute Macuplex ™ D-34C 27  3 minutes Water rinse Ambient 1 minuteMacuplex ™ 9369 48  2 minutes Water rinse Ambient 1 minute Macuplex ™J-64 32  7 minutes Water rinse Ambient 1 minute Dry

The result was no electroless nickel metallization.

EXAMPLE 7

Example 7 illustrates an ABS substrate processed through a non-chromeetch solution, a N,N-dimethylethanolamine post treatment solution and upto the electroless nickel stage:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50  2 minutesWater rinse Ambient 1 minute Etch Same as Example 6 55  6 minutes Waterrinse Ambient  2 minutes Amine treatment 10 g/L N,N- 80 10 minutes dimethylethanolamine Water rinse Ambient 1 minute Acid rinse 2.8M HClAmbient 1 minute Macuplex ™ D-34C 27  3 minutes Water rinse Ambient 1minute Macuplex ™ 9369 48  2 minutes Water rinse Ambient 1 minuteMacuplex ™ J-64 32  7 minutes Water rinse Ambient 1 minute Dry

The result was full electroless nickel metallization.

EXAMPLE 8

Example 8 illustrates an ABS substrate processed through a non-chromeetch solution, a diethylene triamine post treatment solution and up tothe electroless nickel stage:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50  2 minutesWater rinse Ambient 1 minute Etch 0.1M AgNO₃ 55 12 minutes  6M HNO₃ 3MH₂SO₄ Water rinse Ambient  2 minutes Amine treatment 15 g/L diethylene50  5 minutes triamine Water rinse Ambient 1 minute Acid rinse 2.8M HClAmbient 1 minute Macuplex ™ D-34C 27  3 minutes Water rinse Ambient 1minute Macuplex ™ 9369 48  2 minutes Water rinse Ambient 1 minuteMacuplex ™ J-64 32  7 minutes Water rinse Ambient 1 minute Dry

The result was full electroless nickel metallization.

EXAMPLE 9

Example 9 illustrates an ABS substrate processed through a non-chromeetch solution, a polymeric quaternary amine post treatment solution andup to the electroless nickel stage:

Temperature Process Details (° C.) Time ND7 Supreme ™ 50 2 minutes Waterrinse Ambient 1 minute  Etch Same as Example 8 50 6 minutes Water rinseAmbient 2 minutes Amine treatment 15 g/L Mirapol WT, a 50 5 minutespolymeric quaternary amine available from Rhodia Water rinse Ambient 1minute  Acid rinse 2.8M HCl Ambient 1 minute  Macuplex ™ D-34C 27 3minutes Water rinse Ambient 1 minute  Macuplex ™ 9369 48 2 minutes Waterrinse Ambient 1 minute  Macuplex ™ J-64 32 7 minutes Water rinse Ambient1 minute  Dry

The result was full electroless nickel metallization.

1. A method of treating a plastic substrate to accept electrolessplating thereon, the method comprising the steps of: a) etching asurface of the plastic substrate by contacting the plastic substratewith an acidic solution containing nitrate ions; b) contacting theetched plastic substrate with a conditioning solution comprising anaqueous solution comprising a quaternary amine; c) activating theplastic substrate; and d) contacting the activated plastic substratewith an electroless metal plating solution to deposit metal thereon. 2.The method according to claim 1, wherein the acidic solution comprisesoxidizing metal ions, wherein the oxidizing metal ions are selected fromthe group consisting of silver, manganese, cobalt, cerium andcombinations thereof.
 3. The method according to claim 2, wherein theacidic solution comprises silver nitrate and nitric acid.
 4. The methodaccording to claim 3, wherein the acidic solution comprises a wettingagent.
 5. The method according to claim 1, comprising the step ofimmersing the plastic substrate into an acid rinse after step (b) andbefore step (c).
 6. (canceled)
 7. (canceled)
 8. (canceled)
 9. (canceled)10. (canceled)
 11. (canceled)
 12. (canceled)
 13. (canceled)
 14. Themethod according to claim 1, wherein the quaternary amine comprises apolymeric quaternary amine having the general formula:

wherein: R¹, R², R³ and R⁴ independently can be the same or differentand may be selected from —CH₃, —CH₂CH₃, —CH(CH₃)₂ or —CH₂CH₂OH; R⁵ is—CH₂CH₂—, —CH₂CH₂CH₂—, —CH₂CH₂CH₂CH₂—, —CH₂CHOHCH₂— or —CH₂CH₂OCH₂CH₂; Xand Y can be the same or different and are selected from Cl, Br, and I;v and u can be the same or different and each can be 1 to 7; and n is 2to about
 200. 15. The method according to claim 14, wherein in thepolymeric quaternary amine, R¹, R², R³ and R⁴ are each CH₃; R⁵ is—CH₂CH₂OCH₂CH₂; v and u are 3; X and Y are Cl; and n is an average ofabout
 6. 16. The method according to claim 1 wherein the concentrationof the amine and/or the ammonia in the conditioning solution is betweenabout 5 and about 100 g/L.
 17. The method according to claim 16, whereinthe concentration of the amine in the conditioning solution is betweenabout 10 and about 50 g/L.
 18. The method according to claim 1, whereinthe conditioning solution has a pH of between about 0 to about
 14. 19.The method according to claim 18, wherein the conditioning solution hasa pH of between about 6 and about
 12. 20. The method according to claim1, wherein the step of activating the etched and conditioned plasticsubstrate comprises contacting the plastic substrate with an activationsolution comprising palladium.
 21. The method according to claim 1,further comprising the step of contacting the activated plasticsubstrate with an acid treatment prior to step (d).
 22. The methodaccording to claim 1, wherein the electroless metal plating solutioncomprises electroless nickel.
 23. The method according to claim 1,wherein the plastic substrate comprises anacrylonitrile/butadiene/styrene copolymer or anacrylonitrile/butadiene/styrene copolymer blended with polycarbonate.24. The method according to claim 2, wherein the oxidizing metal ionsare produced by electrochemical oxidation.
 25. The method according toclaim 24, wherein the oxidizing metal ions comprise Ag²⁺.